The present invention relates to calibration systems, and more particularly to a logarithmic amplifier calibrator which provides a more accurate logarithmic display for an electronic instrument requiring a precise logarithmic amplifier without using any internal or external standard in the form of a precision attenuator or precision gain step.
Precision instruments which use logarithmic amplifiers require a precise calibration of the logarithmic amplifiers to provide the greatest measurement accuracy. Generally this requires precise test instruments in a laboratory environment, i.e., precision signal generators and step attenuators. As shown in FIG. 1 a test signal having an exponential envelope is input to the logarithmic amplifier of a precision instrument to be calibrated and the output is observed. The output for an ideal logarithmic amplifier is a ramp output having a smooth slope. However in actuality there is some ripple which appears on the slope. The logarithmic amplifier output is compared with an inverse ramp signal from the signal generator by adding the output with the inverse ramp to produce a constant d.c. output component. The logarithmic amplifier is then adjusted until the ringing is minimized. Then the exponential signal is removed and a known precision amplitude signal is input via a precision attenuator. The logarithmic amplifier is now adjusted to provide the desired gain characteristics. This calibration procedure is labor intensive and can only be done at a test bench having the necessary test equipment.
What is desired is a method for precisely calibrating logarithmic amplifiers in an electronic instrument in the field without the requirement for external test equipment.